A development of optical design and optical component manufacturing technologies over decades has enabled downsizing of zoom lenses of which aperture ratio is still kept as low as 2.8. Specifically, there have been developed a type of zoom lenses having four of lens groups; that is, the leading or foremost 1st group of lens pieces of positive refractive power, the succeeding 2nd group of lens pieces of negative refractive power, the 3rd group of lens pieces of positive refractive power, and the trailing or rearmost 4th group of lens pieces of positive refractive power (e.g., see Patent Document 1 listed below).
Another type of compact zoom lenses developed so far are compatible with digital cameras that are typically provided with an imaging device generally referred to as APS-C imaging sensor and dimensioned roughly six tenth as large as 35 mm format, which also has four lens groups; that is, the leading or foremost 1st group of lens pieces of positive refractivity, the succeeding 2nd group of lens pieces of negative refractivity, the 3rd group of lens pieces of positive refractivity, and the trailing or rearmost 4th group of lens pieces of positive refractivity where at least the 1st, 3rd and 4th lens groups among all are simply displaced toward an object along the optical axis so that the 1st and 2nd lens groups at the telephoto end are split farther away from each other than they are at the wide-angle end while the 2nd and 3rd lens groups at the telephoto end come closer to each other (e.g., see Patent Document 2 listed below). Such zoom lenses satisfy conditions as depicted in the formula as follows:4.0<f1/fw<7.02.0<f3/fw<5.0where i is an ordinal number used to show in which position the lens groups are arranged one after another from the object to the imaging plane, fi is a focal length of the i-th lens group, and fw is the focal length of the whole optical system at the wide-angle end, and the zoom lenses are 3.42 in zoom ratio and range approximately from 4.1 to 5.9 in aperture ratio.
In addition, still another type of the compact zoom lenses with the vibration compensating mechanism have been developed, which provide a long back focus suitable to digital single-lens reflex cameras, are as great as ×10 in zoom ratio, and provides a field angle of 70 degrees or even wider at the wide-angle end (e.g., see Patent Document 3 listed below). Such zoom lenses, which comprise the leading or foremost 1st lens group G1 of positive refractivity, the succeeding 2nd lens group G2 of negative refractivity, the 3rd lens group G3 of positive refractivity, and the trailing 4th lens group G4 of positive refractivity, vary power by altering a distance between the adjacent ones of the lens groups. The 3rd lens group G3 has the leading subset G3F of the lens pieces of positive refractive power located closer to the object and the trailing subset G3R of the lens pieces of negative refractive power, and the trailing subset G3R alone are displaced in directions perpendicular to the optical axis to compensate for deviation of the resultant image formed in the plane. At least one major surface of any of the lens pieces in the trailing subset G3R is aspherical so that its positive refractive power is enhanced or its negative refractive power is diminished as it is farther apart from the optical axis toward the periphery in comparison with a spherical surface curved with a radius of paraxial curvature where such an aspherical surface is shaped, meeting requirements defined in predetermined conditional formula.
Similarly, there have been disclosed a similar type of the zoom lenses with the vibration compensating mechanism, which provide a long back focus applicable to digital single-lens reflex cameras as well, vary aperture ratio from 3.52 at the wide-angle end to 5.08 at the intermediate zoom range and further to 5.88 at the telephoto end, and provide a field angle of 70 degrees or even greater at the wide-angle end (e.g., see Patent Document 4). Such zoom lenses, which have four of the lens groups including the leading or foremost 1st lens group G1 of positive refractivity located closer to the object, the succeeding 2nd lens group G2 of negative refractivity, the 3rd lens group G3 of positive refractivity, and the trailing 4th lens group G4 of positive refractivity, have the 1st and 2nd lens groups G1,G2 split farther away from each other, have the 2nd and 3rd lens groups G2, G3 come closer to each other, and the 3rd and 4th lens groups G3, G4 do not stay at a fixed distance from each other as the zoom lenses vary power from the wide-angle end to the telephoto end. The 3rd lens group G3 has the leading subset G31 of the lens pieces of positive refractive power closer to the object and the trailing subset G32 of the lens pieces of negative refractive power, and the trailing subset G32 alone are displaced in directions perpendicular to the optical axis to compensate for the undesirably deviated image plane due to vibrations by tremors of the user's hand(s) so as to meet conditions as expressed in predetermined formula.
Another similar type of the zoom lenses with the vibration compensating mechanism have been disclosed, which additionally have an anti-vibration mechanism functioning to form an adjusted image as a result of vibration compensation (e.g., see Patent Document 5). Such zoom lenses have the leading lens group L1 of positive refractivity located closer to the object and the remaining lens groups succeeding to the same as well as the trailing or rearmost lens group L4 of negative refractivity behind the aperture stop SP closer to the imaging plane, which are all varied in distance between the adjacent ones of the lens groups during the zooming. The lens group L4 is comprised of two lens components of negative refractive power one of which closer to the aperture stop SP is displaced in directions perpendicular to the optical axis to alter a position where the image is to be formed.
Such zoom lenses meet requirements for lens optics as bright as 2.8 in aperture ratio suitable for the state-of-the-art digital camera zoom lenses, which contemplate renovated features in view of anti-vibration and anti-blur as well as of widening a variety of pictorial representations, allowing for a reduced depth of focus of such bright lens optics with a reduced aperture ratio that is suitable for the pictorial representation like portrait.